Industrial robots perform a variety of tasks involving the movement and manipulation of various objects. A typical industrial robot as used, e.g., in a manufacturing environment, may have one or more arms equipped with grippers that allow the robot to pick up, transport, and manipulate objects. A key mechanical requirement for industrial is the ability to generate large but precise forces and torques while maintaining overall control stability. These torques and forces are generated by actuators, i.e., motors responsive to control signals to apply a commanded torque, which is transmitted mechanically to a load either directly (where rotational actuation is required) or via a linear conversion element, such as a lead screw (when linear force is required).
Stiff actuators can exert large forces from small joint displacements, and permit high-bandwidth force control and precise position control. But stiffness makes force control difficult. Because of the importance of force control in robotic applications, stiffness and the attendant bandwidth is typically sacrificed to achieve better force control. One approach is to utilize an elastic element in series with the actuator. Elasticity has the effect of making the force control easier, as larger deformations are needed to exert a given force relative to a stiff actuator. In effect, the elasticity allows force to be controlled via position rather than directly, which improves accuracy and stability, and reduces noise.
Designing series elastic elements for robotic applications can be challenging due to space constraints, the need to withstand large and repeated applied torques without slippage or wander, and the need for repeatable but economical manufacture. In a rotational elastic element, for example, the design must incorporate components with sufficient length to provide the desired elasticity (since stiffness varies inversely with the cube of a component's length), but must also provide a secure mounting frame to avoid slippage. Because the frame typically defines the outer envelope of the elastic element, it imposes a limit on the amount of internal length that may be employed.